The present invention relates to a medium transportation apparatus and an image forming apparatus having the medium transportation apparatus.
Conventionally, an image forming apparatus such as a printer, a copier, a facsimile, and the likes is provided with a medium transportation apparatus for transporting a recording medium to an image forming unit. The medium transportation apparatus may be configured to switch between one mechanism (a medium transportation mechanism) for picking up a plurality of media retained in a medium tray one by one and transporting the medium to the image forming unit and another mechanism (an MPT transportation mechanism) for picking up a plurality of media retained in an MPT (Multi Purpose Tray) one by one and transporting the medium to the image forming unit (refer to Patent Reference). Patent Reference: Japanese Patent Publication No. 2005-212999
FIG. 6(A) is a schematic view showing a configuration of a conventional medium transportation apparatus. As shown in FIG. 6(A), the conventional medium transportation apparatus includes a drive source gear 53 for transmitting a drive force from a drive source (not shown); an idle gear 54 engaging the drive source gear 53; a sun gear 55 engaging the idle gear 54 and a planetary gear 56; the planetary gear 56 for transmitting the drive force from the sun gear 55 to one of a register roller gear 60 and an idle gear 57 of a medium transportation mechanism through engaging therewith according to a rotational direction of the sun gear 55; a reduction gear 58 engaging the idle gear 57; an MPT sheet supply roller gear 59 engaging the reduction gear 58; a register roller 75; and an MPT sheet supply roller 77. A recording medium is represented with a hidden line in FIG. 6(A).
FIG. 6(B) is a schematic perspective view showing a configuration surrounding the planetary gear 56 of the conventional medium transportation apparatus. As shown in FIG. 6(B), in addition to the components shown in FIG. 6(A), the conventional medium transportation apparatus includes a pressing plate member 62 disposed in the planetary gear 56; a compression spring 61 disposed between the planetary gear 56 and the pressing plate member 62 for pushing the pressing plate member 62 outwardly; brackets 63 for holding the planetary gear 56; and guide holes 64 formed in the brackets 63 for guiding a rotational shaft of the planetary gear 56.
In FIG. 6(A), when the drive source gear 53 rotates in the right direction, the sun gear 55 rotates in the right direction through the idle gear 54, thereby transmitting the drive force of the left rotation to the planetary gear 56. At this moment, the compression spring 61 pushes the pressing plate member 62 disposed in the planetary gear 56 against the bracket 63, thereby causing a frictional force therebetween. Accordingly, the rotational shaft of the planetary gear 56 moves in the guide hole 64 in the right direction.
When the rotational shaft of the planetary gear 56 moves in the guide hole 64 in the right direction, the planetary gear 56 engages the idle gear 57. Accordingly, the drive force transmitted from the drive source gear 53 is transmitted to the MPT sheet supply gear 59 through the idle gear 54, the sun gear 55, the planetary gear 56, the idle gear 57, and the reduction gear 58, thereby rotating the MPT sheet supply roller 77. As a result, the recording medium retained in the MPT is separated and transported one by one.
When the drive source gear 53 rotates in the left direction, the sun gear 55 rotates in the left direction through the idle gear 54, thereby transmitting the drive force of the right rotation to the planetary gear 56. At this moment, the rotational shaft of the planetary gear 56 moves in the guide hole 64 in the left direction. Accordingly, the planetary gear 56 engages the register roller gear 60. As a result, the drive force transmitted from the drive source gear 53 is transmitted to the register roller gear 60 through the idle gear 54, the sun gear 55, and the planetary gear 56, thereby rotating the register roller 60. Accordingly, the recording medium is transported to the image forming unit.
As shown in FIG. 6(A), the compression spring 61 pushes the pressing plate member 62 disposed in the planetary gear 56 against the brackets 63, thereby generating pressing forces Pa′ and Pb′. Accordingly, frictional forces Fa′ and Fb′, i.e., products of the pressing forces Pa′ and Pb′ and coefficients of friction μa′ and μb′, respectively, are generated at points A′ and B′, respectively. As a result, when the sun gear 55 rotates, the rotational shaft of the planetary gear 56 moves in the guide hole 64.
It is supposed that a distance between an engagement point between the planetary gear 56 and the sun gear 55 to the point A′ of the frictional force Fa′ is La′, and a distance between the engagement point to the point B′ of the frictional force Fb′ is Lb′. According to a moment relationship, when the frictional force Fb′ is less than La′/Lb′ times of the frictional force Fa′ (Fb′<La′/Lb′×Fa′), the rotational shaft of the planetary gear 56 moves in a direction of a force F′ applied from the sun gear 55, thereby switching the gear engaging the planetary gear 56.
In the conventional medium transportation apparatus, the compression spring 61 tends to push the pressing plate member 62 at a larger force at an end portion thereof. Further, the pressing plate member 62 tends to deform. Accordingly, the pressing forces Pa′ and Pb′ tend to fluctuate. Further, the pressing plate member 62 does not abut against the bracket 63 at a constant location, rather various locations along a circle. Accordingly, when the planetary gear 56 switches the gear, the frictional forces Fa′ and Fb′ always vary.
According to the moment relationship around the rotational shaft of the planetary gear 56, when the frictional force Fb′ becomes equal to La′/Lb′ times of the frictional force Fa′ (Fb′=La′/Lb′×Fa′), the planetary gear 56 stops rotating, thereby making it difficult to switch the gear. Further, when the frictional force Fb′ becomes greater than La′/Lb′ times of the frictional force Fa′ (Fb′>La′/Lb′×Fa′), the rotational shaft of the planetary gear 56 moves in a direction opposite to the direction of the force F′ applied from the sun gear 55. As a result, the planetary gear 56 switches the gear at various timings or a delayed timing, and it is difficult to sufficiently switch the gear.
In view of the problems described above, an object of the present invention is to provide a medium transportation apparatus and an image reading apparatus having the medium transportation apparatus, in which a planetary gear stably moves in a direction of a force applied from a sun gear even when a frictional force or a reaction force varies.
Further objects and advantages of the invention will be apparent from the following description of the invention.